1. Field of the Invention
The present invention relates to an electrolyte-electrode joined assembly comprising a first electrode which functions an one of an anode and a cathode, a second electrode which functions as the other of the anode and the cathode, and a solid electrolyte which is interposed between the first electrode and the second electrode, and a method for producing the same.
2. Description of the Related Art
As shown in FIG. 6, a cell, which is provided with an electrolyte-electrode joined assembly 4 comprising a solid electrolyte 3 interposed between an anode 1 and a cathode 2, is one type of an electrochemical cell such as a fuel cell and an oxygen sensor.
For example, in a fuel cell, materials for the anode 1 include, for example, a cermet containing Ni and stabilized zirconia (YSZ) doped with about 8 mole % of Y2O3 in a weight ratio of 1:1. Materials for the solid electrolyte 3 include, for example, YSZ, and examples of materials for the cathode 2 include a perovskite type oxide such as LaMnO3.
The electrolyte-electrode joined assembly 3 as described above is manufactured as follows. At first, NiO powder and YSZ powder are blended in a weight ratio of 1:1, followed by being pulverized and mixed with a wet ball mill or the like to prepare a slurry.
Subsequently, the slurry is formed into a film, for example, by the doctor blade method or the like. Thus, the anode 1 composed of the mixture of NiO and YSZ is prepared.
Meanwhile, YSZ powder is pulverized with a wet ball mill or the like to prepare a slurry. The slurry is formed into a film, for example, by the doctor blade method or the like in the same manner as described above. Thus, the solid electrolyte 3 composed of YSZ is prepared.
The solid electrolyte 3 manufactured as described above is stacked on one end surface of the anode 1, and both the anode 1 and the solid electrolyte 3 are simultaneously sintered in this state. Accordingly, the anode 1 and the solid electrolyte 3 are joined together.
Subsequently, a slurry of LaMnO3 is applied to one end surface of the solid electrolyte 3 to form a film, for example, by the screen printing method. When the slurry is heated together with the solid electrolyte 3 and the anode 1 in the air atmosphere, the electrolyte-electrode joined assembly 4 is obtained, in which the cathode 2 is fired on the solid electrolyte 3, and the solid electrolyte 3 is interposed between the anode 1 and the cathode 2.
Various investigations have been made about such an electrolyte-electrode joined assembly 4 in order to improve the performance. For example, Japanese Laid-Open Patent Publication No. 6-295730 suggests that an anode has a two-layered structure comprising a lower layer composed of NiO and an upper layer composed of a mixture of NiO and YSZ in order to improve the collecting function of the anode. Meanwhile, Japanese Laid-Open Patent Publication No. 2003-173802 suggests that a reaction-preventive layer composed of Ce1-xLnxO2-δ, which has a porosity of not more than 25%, is provided between a solid electrolyte and at least one of an anode and a cathode. According to Japanese Laid-Open Patent Publication Nos. 6-295730 and 2003-173802, the resistance value of the electrolyte-electrode joined assembly is lowered.
In both of the fuel cells as described above, a fuel gas containing hydrogen is supplied to the anode 1, while an oxygen-containing gas containing oxygen is supplied to the cathode 2. In particular, the fuel gas is moved toward the solid electrolyte 3 through pores of the anode 1. The oxygen contained in the oxygen-containing gas combines with the electron in the cathode to produce oxide ion (O2−). The hydrogen contained in the fuel gas combines with the oxide ion (O2−) having arrived at YSZ contained in the anode from the cathode via the solid electrolyte to produce steam and electrons. The steam is moved to one end surface of the anode through pores of the anode, and the steam is finally discharged from the end surface. Accordingly, the anode 1 is required to be a porous member including pores at a predetermined ratio in order to quickly diffuse the fuel gas supplied to the anode 1 and the produced steam.
As shown in FIG. 7, the anode 1 as the porous member includes recesses 6 which are formed by the pores 5 having openings at the end surface facing the solid electrolyte 3, and projections 7 which bulge out of the end surface. Therefore, when the slurry of the solid electrolyte 3 is thinly applied, then either the solid electrolyte 3 enters the recesses 6, or the solid electrolyte 3 is formed in a state of being stacked on the projections 7. Therefore, the depressions (recesses 8) and the bulges (projections 9) appear on the solid electrolyte 3. If the anode 1 and the solid electrolyte 3 are sintered in this state, stress is concentrated on the recesses 8 and the projections 9 of the solid electrolyte 3, because the coefficient of thermal expansion differs between the recesses 8 and the projections 9. As a result, cracks appear in the solid electrolyte 3, therefore inhibiting the oxide ion conduction, and the power generation characteristics of the fuel cell or the like are deteriorated.
As shown in FIG. 8, the recesses 6 may be filled and the projections 7 may be buried by increasing the thickness of the solid electrolyte 3. However, in this case, the volume resistance or the volume resistivity of the solid electrolyte 3 is increased, and the power generation characteristics of the fuel cell or the like are deteriorated. Further, it is difficult to miniaturize the electrolyte-electrode joined assembly 4, because the thickness of the electrolyte-electrode joined assembly 4 is increased.
No attempts have been made to dissolve the inconvenience concerning the above so for.